Titus Tatius, the Sabini's leader, gave Tarpeia a choice of gifts, if she would lead his army into the Capitol. She asked for what the Sabini were wearing on their left hands, namely gold rings and bracelets. Titus deceitfully promised these, and she led the army into the citadel, where Titus ordered that she be buried under shields, because already they'd had these on their left arms. In such a way treacherous betrayal is revenged by a swift punishment.

Why is habuerant plural? I thought it was just Tarpeia, and it should be nam et ea in laevā habuerat "because she'd already had these on her left hand.". What, then is the subject of habuerant? What have I missed?

Tarpeia asked for the rings and bracelets the soldiers had, but instead she gets the shields they were holding in their left hands. They were thrown upon her, and killing her that way. So the soldiers are the subject of habuerant.